The present invention relates to a copier, printer, facsimile apparatus or similar Image forming apparatus and an image transferring device therefore. More particularly, the present invention relates to an image forming apparatus of the type transferring a toner image from an intermediate image transfer body, photoconductive element or similar image carrier to a recording medium, and an image transferring device therefor.
Generally, an image forming apparatus is implemented as an electrophotographic copier, a printer, a facsimile apparatus or a multiplex machine having at least two of their functions. One of conventional image transferring devices for use in the image forming apparatus includes a pair of image transfer members sandwiching an image carrier and forming a nip for image transfer therebetween. This type of image transferring device separates a recording medium moved away from the above nip from the image carrier by using a curvature. However, to separate the recording medium by means of a curvature, the surface of the image carrier moved away from the nip must be steered to a degree capable of implementing the curvature. Therefore, the curvature separation scheme is not applicable to an image forming apparatus involving various limitations on layout, configuration of the image carrier, etc.
When the above curvature separation is not practicable, a discharger for separation may be located on a path along which the surface of the image carrier moved away from the image transfer nip linearly moves. This, however, brings about another problem that if the recording medium moved away from the image transfer nip is not immediately separated from the image carrier, toner transferred to the medium is apt to again deposit on the image carrier. Such redeposition of the toner on the image carrier would result in a defective image. This is particularly true with the trailing edge of the recording medium because discharge for separation is not always applied to the trailing edge due to the ON/OFF timing of a voltage assigned to the discharger.
Another problem with the conventional image transferring device is that when the trailing edge of the recording medium moves away from the nip or image transfer region, it is apt to jump up and contact a structural element included in the image forming apparatus. This disturbs the toner image carried on the image carrier, but not fixed, or causes toner to deposit on and smear the trailing edge of the recording medium. Particularly, when the recording medium has a high resistance or a low water content, the medium is easy to charge and causes its trailing edge to be easily electrostatically attracted by the image carrier and jump up.
To solve the above jump problem, a voltage opposite in polarity to the image transfer voltage may be applied to the image transferring device when the trailing edge of the recording medium moves away from the nip, as proposed in the past. Alternatively, a voltage applying device may be interposed between the image transferring device and the separating device for applying a charge opposite in polarity to the image transfer voltage to the trailing edge of the recording medium moved away from the nip, as also proposed in the past. However, the former scheme has a problem that the toner of the toner image transferred to the recording medium is partly electrostatically returned to the image carrier, degrading the quality of the toner image. The latter scheme has a drawback that the voltage applying means makes the construction of the image forming apparatus sophisticated and increases the cost.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-234577 and 9-43996.